Caption: Small intestine. Coloured scanning electron micrograph (SEM) showing the surface of villi in the small intestine. Villi are folds in the wall of the intestine. They project into the lumen and greatly increase the absorptive surface of this part of the digestive system. Once food has been digested by stomach acid and enzymes secreted into the duodenum, it is ready to be absorbed by the small intestine. Each villus has capillaries into which food is absorbed. The villi also secrete a mucus to aid movement of food through the gut. For the reason of food absorption, the small intestine is the longest region in the gut. Magnification unknown.